What's a Collection? In Explore, a Collection is a list reported in a variable. You can see the contents of a Collection, but you can't modify it. Here's some examples of Collections in Explore:
The similarity in all of these examples is that they report lists. (all sprites) reports a list of all of the sprites in the program, for instance. You can get information from this list:
A final trick: If you have Explore 3.0 or later, you can see everything that's in a Collection. Just double-click a Collection block and see its contents appear!